The invention relates to a cathode ray tube having an electron source, which comprises a cathode structure which contains an electron-emitting material at an end portion and in which a heating element of bifilarly wound wire having primary and secondary turns is accommodated, said wire being provided, except in the vicinity of the ends of the wire facing away from the end portion, with an electrically insulating layer having a radius r, and said uncovered ends each being connected to an electric conductor by means of an electric connection.
The invention further relates to a heating element for use in a cathode structure of an electron source in a cathode ray tube.
Cathode ray tubes in which cathode structures comprising heating elements are used in electron sources are, for example, (flat-panel) display devices for displaying monochromatic or color images, camera tubes and oscilloscope tubes. Examples of electron sources are so-called impregnated cathodes or so-called oxide cathodes.
A cathode structure of the type mentioned in the opening paragraph is known from the brochure "Quick-Vision CTV Picture Tube A66-410X" by L. J. G. Beriere and A. J. van IJzeren (Philips Product Note, 1973). In this brochure, a description is given of a tubular cathode structure in an electron gun for use in a cathode ray tube, having a layer of an electron-emitting material at an end portion to emit electrons. A heating element which serves to heat the electron-emitting material is arranged in the cathode structure. Said heating element comprises a wire which is bifilarly wound in the form of a double helix and which has primary and secondary turns. Said wire is provided with an electrically insulating layer.
Under specific conditions, the life of the cathode ray tube is governed by the life of the heating element. This applies in particular to cathodes which are operated at relatively high temperatures and in which, simultaneously, a low power is dissipated (for example in low-power impregnated cathodes).